[PATCH] Cygwin: chown: make sure ctime gets updated when necessary

[Ken Brown]

On 1/25/2021 1:57 PM, Corinna Vinschen via Cygwin-patches wrote:
> On Jan 25 12:24, Ken Brown via Cygwin-patches wrote:
>> Following POSIX, ensure that ctime is updated if chown succeeds,
>> unless the new owner is specified as (uid_t)-1 and the new group is
>> specified as (gid_t)-1.  Previously, ctime was unchanged whenever the
>> owner and group were both unchanged.
>>
>> Aside from POSIX compliance, this fix makes gnulib report that chown
>> works on Cygwin.  This improves the efficiency of packages like GNU
>> tar that use gnulib's chown module.  Previously such packages would
>> use a gnulib replacement for chown on Cygwin.
>> ---
>>   winsup/cygwin/fhandler_disk_file.cc | 10 +++++++++-
>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/winsup/cygwin/fhandler_disk_file.cc b/winsup/cygwin/fhandler_disk_file.cc
>> index 07f9c513a..72d259579 100644
>> --- a/winsup/cygwin/fhandler_disk_file.cc
>> +++ b/winsup/cygwin/fhandler_disk_file.cc
>> @@ -863,6 +863,7 @@ fhandler_disk_file::fchown (uid_t uid, gid_t gid)
>>     tmp_pathbuf tp;
>>     aclent_t *aclp;
>>     int nentries;
>> +  bool noop = true;
>>   
>>     if (!pc.has_acls ())
>>       {
>> @@ -887,11 +888,18 @@ fhandler_disk_file::fchown (uid_t uid, gid_t gid)
>>   				    aclp, MAX_ACL_ENTRIES)) < 0)
>>       goto out;
>>   
>> +  /* According to POSIX, chown can be a no-op if uid is (uid_t)-1 and
>> +     gid is (gid_t)-1.  Otherwise, even if uid and gid are unchanged,
>> +     we must ensure that ctime is updated. */
>>     if (uid == ILLEGAL_UID)
>>       uid = old_uid;
>> +  else
>> +    noop = false;
>>     if (gid == ILLEGAL_GID)
>>       gid = old_gid;
>> -  if (uid == old_uid && gid == old_gid)
> 
> Basically ok, but why not just
> 
>       if (uid == ILLEGAL_UID && gid == ILLEGAL_GID)
> 
> instead of the noop var?

I went back and forth on that.  Following your suggestion, the code looks like this:

   if (uid == ILLEGAL_UID && gid == ILLEGAL_GID)
     {
       ret = 0;
       goto out;
     }
   if (uid == ILLEGAL_UID)
     uid = old_uid;
   if (gid == ILLEGAL_GID)
     gid = old_gid;

I was trying to avoid checking uid == ILLEGAL_UID and gid == ILLEGAL_GID twice. 
  But on second thought, it's probably silly to worry about that.  The code is 
cleaner without the noop variable.

Thanks.

Ken